.gamma.-Fe.sub.2 O.sub.3 magnetic pigments have coercivity values of up to about 31 kA/m and ferrite magnetic pigments have coercivity values of up to about 35 kA/m. Magnetic pigments having substantially higher coercive field strengths are increasingly required for magnetic signal recording in audio, video and data storage systems to increase the storage density. Various processes are known for increasing the coercive field strength of magnetic iron oxides.
According to German 2,903,593, pigments with higher coercivity values are obtained from .alpha.-FeOOH precursor compounds which are doped with cobalt. A coercive field strength of about 50 kA/m is obtained with 1% cobalt and and FeO content of 20%. The disadvantage of these pigments, however, lies in their lower printing attenuation, their insufficient magnetic stability and the high temperature dependence of the coercivity.
Another method of obtaining high coercive field strengths has been described in German 2,235,383, in which a layer of cobalt ferrite is left to crystallize epitactically on a core of magnetic iron oxide in a strongly alkaline medium. These pigments have better values for printing attenuation and magnetic stability than the cobalt-doped pigments but are not entirely satisfactory in their control panel distribution, the dependence of their coercivity on the temperature, and the ratio of erasure in tapes. Pigments with more satisfactory properties can be obtained by applying several layers of compounds containing cobalt and/or iron to the core, as described in German 3,520,210.
In German 3,344,299 there is described a process for the preparation of a magnetic iron oxide powder containing cobalt, in which the iron oxide starting material is first coated with an iron-(II) compound and then with a cobalt compound. The magnetic properties are thereby improved.
It is an object of the present invention to provide magnetic iron oxide pigments with higher coercive field strength values which do not have the disadvantages described above of the magnetic iron oxide pigments of the prior art.